Ocular Myasthenia Gravis Flashcards
what is myasthenia gravis
an autoimmune disorder characterized by weakness and fatiguability of skeletal muscles due to dysfunction of neuromuscular junction
what is myasthenia gravis
an autoimmune disorder characterized by weakness and fatiguability of skeletal muscles due to dysfunction of neuromuscular junction
what is ocular MG
symptoms of MG isolated to the levator palpebrae superioris, orbicularis oculi, and the oculomotor muscles
- over half present with ptosis, diplopia, or both & no signs of weakness elsewhere
what is the pathophysiology of OMG
unknown why isolated to levator palpebrae
- more likely to seronegative for AChR-Ab
- EOM are unique in their expression of fetal AChR at the NMJ
- junctional folds of muscle end plates are sparse in the EOM and levator muscles, perhaps producing a lower safety factor for neuromuscular transmission
- complement regulatory genes are expressed differently in EOMs, perhaps reducing protective mechanisms to complement-mediated tissue injury
what are the clinical features of OMG
characterized by ptosis and oculomotor paresis
- some have mild orbicularis oris weakness
describe ptosis in OMG
unilateral and asymmetric on presentation - historical pattern of alternating from one side to the other examination of eyelid fatigue - ptosis fluctuating throughout exam - upgaze - rapid fatigue after rest - eyelid 'curtaining' - 'rest test' - ice pack test
describe diplopia in OMG
binocular with ophthalmoparesis
- if medial or lateral recti muscles: binocular & horizontal
- if superior or inferior recti or oblique: vertical/diagnoal component
fluctuation in degree of diplopia or the direction of gaze elicits diplopia suggests fatiguable ocular motor paresis
is the pupil involved in OMG
generally not
what’s the differential diagnosis for OMG
thyroid ophthalmopathy
- lack of ptosis
- presence of proptosis, lid retraction, lid lag, periorbital edema
chronic progressive external ophthalmoplegia/kearns-sayre syndrome
- mitochondrial disorder that produces progressive, generally symmetric ophthalmoparesis and ptosis
- slow saccades (vs normal in OMG), isolated to EOM
- generalized muscle weakness, cerebellar disease, retinal degeneration
muscular/oculopharyngeal dystrophy
brainstem and motor CN pathology
what are the diagnostic tests for OMG
clinical, with confirmatory testing
- serum Ab studies (AChR-Ab neg; some MuSK-Ab +; low density lipoprotein receptor-related protein 4-Ab +)
- repetitive nerve stimulation (decrement in amplitude of compound muscle action potentials after repetitive stimulation)
- single fiber EMG (abnormal neuromuscular transmission by measuring temporal variability in firing of adjacent motor nerve fibers from a single motor neuron unit [jitter])
what is the prognosis in OMG
of patients presenting with OMG, 2/3 will develop signs/symptoms of extremity weakness and other bulbar muscle weakness
- only 1/3 will have pure OMG
what is the treatment of OMG
symptomatic
- eye patch, prism lens
immunomodulatory
- anticholinesterase agents [pyridostigmine] rarely results in resolution of ocular symptoms
- immunosuppressive agents (start at low dose, work up with steroids)
- steroid-sparing agents (second line)
- thymectomy (all patients with thymoma and MG)
- surgery for ptosis and diplopia (only if stable)
what is ocular MG
symptoms of MG isolated to the levator palpebrae superioris, orbicularis oculi, and the oculomotor muscles
- over half present with ptosis, diplopia, or both & no signs of weakness elsewhere
what is the pathophysiology of OMG
unknown why isolated to levator palpebrae
- more likely to seronegative for AChR-Ab
- EOM are unique in their expression of fetal AChR at the NMJ
- junctional folds of muscle end plates are sparse in the EOM and levator muscles, perhaps producing a lower safety factor for neuromuscular transmission
- complement regulatory genes are expressed differently in EOMs, perhaps reducing protective mechanisms to complement-mediated tissue injury
what are the clinical features of OMG
characterized by ptosis and oculomotor paresis
- some have mild orbicularis oris weakness
